1. Field of the Invention
The present invention relates to a method of making an ultra low-carbon cold-rolled steel sheet, capable of preventing the usual upper nozzle, sliding nozzle and immersion nozzle of a tundish ("nozzles") from clogging. Such clogging often occurs during continuous casting of aluminum-killed molten steel due to Al.sub.2 O.sub.3 adhesion to the inner walls of these nozzles. The method of this invention is also capable of minimizing or preventing occurrence of surface defects on the slabs that are produced by the continuous casting process. Such surface defects have, in the past, been due to Al.sub.2 O.sub.3 clusters which inevitably formed in the aluminum-killed molten steel. This invention is also capable for the first time of preventing defects due to Al.sub.2 O.sub.3 in cold-rolled steel sheets produced from such slabs.
2. Description of the Related Art
In manufacturing ultra low-carbon cold-rolled steel sheets, aluminum is generally added to the molten steel after decarburization to decrease the soluble oxygen content in the molten steel, and to increase the yield achievable by Ti and Nb to precipitate and fix carbon and nitrogen in the steel during melting. Aluminum addition is also done in order to prevent blow holes from forming on the surface of the slab during continuous casting. When such aluminum-killed steel is subjected to continuous casting, Al.sub.2 O.sub.3 -based oxides tend to be formed during deoxidation and tend to adhere to the inner walls of the nozzles of the tundish and clog the nozzles. Thus, the available channel for molten metal flow is narrowed and prevents achievement of the desired flow molten metal rate. Further, when pieces of Al.sub.2 O.sub.3 peel off the inner walls of the nozzles, they are captured by coagulated shells in the cast slab, resulting in surface defects in the slab.
Efforts have been made to solve such problems. One has been to blow inert gas such as gaseous argon from the nozzles of the tundish to prevent Al.sub.2 O.sub.3 -based oxide adhesion to the inner walls of the nozzles. However, blown inert gas is captured by coagulated shells in the slab and results in formation of new bubble defects in the slab.
There are other disclosed methods for attempting to reduce Al.sub.2 O.sub.3 adhesion to the inner walls of the nozzles, in which metallic or alloyed calcium such as Ca--Si is added to the molten steel to convert Al.sub.2 O.sub.3 inclusions to CaO-Al.sub.2 O.sub.3 inclusions having lower melting points, as follows:
1) Japanese Unexamined Patent Publication No. 58-154447 discloses a method in which 0.2 to 0.5 kg/t of calcium is added to molten metal in a ladle so as to decrease the melting point of Al.sub.2 O.sub.3 inclusions, in which the resulting CaO-Al.sub.2 O.sub.3 melt rises to the surface of the molten steel and is removed from the ladle. PA1 2) Japanese Unexamined Patent Publication No. 61-276756 discloses a method in which metallic or alloyed calcium is added to aluminum-killed molten metal during a melting or continuous casting step so that 2 to 40 ppm of calcium, which forms CaO-Al.sub.2 O.sub.3 inclusions, remains in the steel.
However, calcium added as metallic or alloyed calcium is converted to CaS and CaO, which results in development of rust on the steel sheet. In particular, rust readily forms at a calcium content of 10 ppm or more in steel. Further, portions of Al.sub.2 O.sub.3 formed during the aluminum-deoxidation process, which did not rise to the surface in the tundish and casting mold, remained as large coagulated inclusions in the cast slab. Such inclusions were captured on the surface layer of the cast slab, resulting in serious surface defects.